The invention relates to a coupling arrangement for coupling a first and a second optical connection, said coupling arrangement comprising
a first optical point of connection for coupling the first optical connection, PA1 a second optical point of connection for coupling the second optical connection, and PA1 a third point of connection, the coupling arrangement further comprising PA1 a first device which is situated between the first and the second point of connection, provided with PA1 a first sub-point of connection which can be coupled to the first point of connection, PA1 a second sub-point of connection which can be coupled to the second point of connection, and PA1 a third and a fourth sub-point of connection for coupling to a second device, and PA1 the second device, which is coupled to the third and fourth sub-point of connection, said second device being provided with an output which is coupled to the third point of connection. PA1 an input which is coupled to the third sub-point of connection of the optical switch, and PA1 a further output which is coupled to the fourth sub-point of connection of the optical switch. PA1 an optical amplifier provided with an input and an output, and PA1 an optical power splitter provided with an input and a first and a second output, PA1 an optical receiver provided with a signal input and an electrical signal output, for receiving an optical signal at the signal input and delivering a corresponding electrical signal at the signal output, PA1 an optical transmitter provided with a signal input and a signal output, for receiving an electrical signal at the signal input and transmitting a corresponding optical signal at the signal output, PA1 in which the signal input of the optical receiver is coupled to the third sub-point of connection of the optical switch, the signal output of the receiver is coupled to the signal input of the optical receiver and to the third point of connection of the coupling arrangement, and the signal output of the optical transmitter is coupled to the fourth sub-point of connection of the optical switch. In an optical network, such a coupling arrangement can also function as an optical network terminating unit.
Such a coupling arrangement is employed in a node of an optical video transport and distribution system as described in M. Yamashita and T. Tsuchiya, "Optical video transport/distribution system with video on demand service", SPIE Vol. 1817 Optical Communications (1992), pp. 12-22 (referred to hereinafter as "Yamashita and Tsuchiya"). This system comprises a distributive network in which a number of network nodes are included in a single optical fibre connection, while a main node is connected to the ends of the optical fibre connection via an optical power splitter, so that a ring-shaped network is formed. The fibre connection is used bi-directionally. In the main node the signals to be distributed, hereinafter referred to as distribution signals, are transmitted as optical signals in both signal transport directions over the optical fibre connection via the optical power splitter. In each network node the distribution signals from both transport directions are amplified, and partly passed on in each of the two transport directions to a subsequent network node, and partly dropped and, via a uni-directional optical connection, further conducted to an optical network terminating unit (ONU) to which a number of subscriber lines are connected. For this drop-and-continue function, each network node is provided with a coupling arrangement of the above-mentioned kind. The coupling arrangement comprises a bi-directional optical amplifier (the first device), implemented as a quadruple port, with which the distribution signals from both signal transport directions are first amplified and are subsequently split into two signal parts in a power coupler. For each signal transport direction one signal part is subsequently transmitted further over the fibre connection in that signal transport direction, and the other (dropped) signal part is conducted to a separate input of two inputs of an optical switch (the second device). The switch, under the control of a control signal, passes the signal part which is presented to one of the two inputs of the switch on to downstream circuits via an output of the switch. The control signal is derived from distribution signals in both transport directions via an optical tapping point placed in the fibre connection at one side adjacent to the bi-directional amplifier in the network node. If the signal in a network node from one of both signal transport directions drops out, as is the case, for example, for a rupture in the fibre connection or on failure of a bi-directional amplifier in a network node elsewhere in the ring-shaped network, then the optical signal over the fibre connection from the other signal transport direction will still be received. This prior art distributive network is therefore self-healing, and the optical switch in the coupling arrangement in each network node has the function of a protective switch. However, this prior art technique has a major disadvantage. Bi-directional amplifiers are very sensitive for reflections and Rayleigh backscattering, so that only a small number of such amplifiers can be applied in a bi-directional fibre connection. This restricts the number of network nodes which can be included in the ring-shaped network and thus the total dimensioning of distributive networks considerably.